The present invention relates to monopolar or bipolar electrolysis cells for the production of metal by the reduction of a metal halide dissolved in at least one molten halide of higher electrodecomposition potential than the metal halide. More particularly, this invention relates to an improvement in such cells for sealing of at least some of the joints of abutment between the various carbonaceous elements and adjacent cell elements located therein in order to avoid the deleterious consequences of physical shifts in the carbonaceous elements during operation of the cell.
A cell in which the present invention may be utilized is described in U.S. Pat. No. 4,179,346 of Das et al. This cell includes an outer steel shell which is lined with refractory brick made of thermally insulating, electrically nonconductive material. In the bottom of the cell is a graphite lined refractory sump for collecting the metal produced in the cell. Above the sump are located a cathode, at least one intermediate bipolar electrode and an anode, all of carbonaceous material, preferably of graphite. These cell components are arranged in a superimposed, spaced relationship defining inter-electrode spaces between the anode and the uppermost electrode, between each pair of intermediate electrodes, and between the lowermost electrode and the cathode. The components are preferably disposed horizontally within a vertical stack. The cathode is preferably supported at each end on the graphite walls of the sump. The remaining electrodes are stacked each above the ones below, with their ends abutting the refractory side walls, in a spaced relationship established by interposed refractory spacers.
The cell of Das et al. operates to produce metal from the electrolytic reduction of the metal chloride dissolved in a molten solvent. This electrolysis takes place in each inter-electrode space to produce chlorine on each anode surface thereof and metal on each cathode surface thereof. Passages are provided through the electrodes in order to aid in the establishment and maintenance of a flow of the chloride-solvent bath through each inter-electrode space to remove the metal produced in each space. The bath flow is directed into, across and out of each inter-electrode space by utilization of the chlorine produced as the lifting gas in a gas lift pump which lifts the lighter bath upwardly while permitting heavier molten metal swept from each inter-electrode space to settle in the sump.
An electrode used in the cell of Das et al. may be comprised of two or more electrode pieces or elements which are fit together in the cell to form an assembled electrode. Alternatively, an electrode may be manufactured as a single element. In either event, the electrodes are comprised of a carbonaceous material, preferably graphite grade carbon, which can be produced from coke derived from coal or petroleum. In making an electrode element, the coke is calcined to drive off volatile impurities, and the calcined coke is blended with a pitch binder to provide a mixture having a pitch content of about 10-30%. This mixture is shaped in the desired size and configuration and baked at about 700.degree.-1600.degree. C. to drive off volatiles from the pitch binder. The baked electrode element is then usually immersed in liquid pitch to impregnate it and increase its density, after which it is again baked at about 700.degree.-1600.degree. C. to drive off volatiles from the pitch. Finally, the carbonaceous material is heated to a temperature of about 2000.degree.-3100.degree. C. for a period sufficient to convert it to graphite.
The present invention may also be utilized in a cell such as is illustrated in U.S. Pat. No. 4,140,594 of Rogers et al. This cell is similar to that illustrated in U.S. Pat. No. 4,179,346 of Das et al., but it includes two stacks of electrodes. It also includes an inner side wall lining of carbonaceous material, which is positioned inside of the refractory brick lining alongside and above the location of the anodes. This lining, as well as the sump lining which is found in both the cell of Rogers et al. and that of Das et al., is formed from carbonaceous elements such as graphite slabs which are fitted into machined recesses in the refractory brick lining.
It has been found that, during the operation of the cells of Das et al. or Rogers et al., the carbonaceous elements may be subject to physical shifting within the cell. This shifting may result from mechanical stresses caused by thermal expansion or chemical reaction, or from buoyancy effects due to the relatively close densities of the carbonaceous elements and the molten electrolyte bath in the cell. This shifting of elements may damage the elements, and it may produce gaps in the joints between such elements and adjacent structures. Thus, gaps may appear between carbonaceous elements assembled together, such as electrode elements assembled to form a single electrode, or between any of the carbonaceous elements and the adjacent cell elements. The appearance of these gaps may lead to penetration therethrough of metal and to deviations from the desired flow of halogen gas produced by the electrolysis process. This could result in re-halogenation of the metal and the combination of the metal with the carbon of the carbonaceous elements on the anodic surfaces of the electrodes. Thus, halides and carbides of the metal may build up a sludge-like formation on the anode surfaces, which would interfere with the efficient operation of the cell by reducing the anode-cathode spacing. Continued accumulation of sludge on the anode surfaces could produce electrical short circuits, thus further impairing electrolysis.
The problem of shifting electrode blocks in electrolytic cells has been previously recognized, as evidenced by the disclosure of U.S. Pat. No. 3,764,509 of Etzel et al. This reference discloses a means for minimizing the effect of mechanical stresses on the carbonaceous cathode in an electrolysis cell used for the production of aluminum. According to this reference, buckling or bulging of such a cathode due to mechanical stresses encountered in the operation of the cell can be minimized by providing the cathode in the form of a set of carbonaceous blocks, each block possessing four lateral surfaces, at least an opposed pair of which are inclined at different angles to the vertical. Thus, the assembled cathode obtains, by virtue of the shapes of its component blocks, a mutual wedging of the blocks against upwardly acting forces.
It is also known that a layer of carbonaceous material such as carbon felt may be utilized to prevent contact between the outer metal shell of an electrolysis cell and the metal produced therein. Such contact can provide an electrical pathway through the cell that bypasses the cathode and therefore does not contribute to the production of metal. Such a use of carbonaceous material is described in U.S. Pat. No. 4,160,715 of Kinosz et al. This reference discloses an improved lining for an electrolytic cell such as the one previously discussed in connection with U.S. Pat. No. 4,140,594 of Rogers et al. This lining includes a layer of a material such as carbon felt within the refractory lining at the bottom of the side walls and under the floor of the sump, where molten aluminum accumulates. This layer is impenetrable by molten aluminum and serves to insure that molten aluminum does not penetrate to the metal shell of the cell.
It is also known that a barrier of carbonaceous material may be provided in an electrolytic cell to protect the thermal insulation in the bottom of the cell from attack by the bath constituents in the cell. Such attack may cause degradation of the physical and insulating properties of the insulation, thereby making it difficult to control the temperature of the bath within the cell. Proper temperature control is essential for efficient operation of the cell.
U.S. Pat. No. 4,175,022 of Vadla et al. describes the provision of a barrier which includes a graphite sheet above the thermal insulation in the bottom of a cell for the production of aluminum by the electrolytic reduction of alumina in a cryolite bath. According to this reference, a sheet formed from expanded graphite in combination with a thin steel underlay gives excellent protection to the insulation beneath against the migration of cryolite, its decomposition products and the bath components.